As I see it, there are three looming threats that all have the potential - individually or combined - to bring the WBTS to its knees, at least in the United States, anyway.
1. Youth retention. The young people aren't buying it. The Watchtower will argue that faithless young people are giving in to demonic persuasion and leaving the Truth. I sumbit to you that this is not the case. Young people are just not stupid. We live in an age where we no longer have to take someone's word for something. Something can be questioned or verified within minutes. Immoral sex isn't causing young people to check out of the Watchtower Hotel, though that is what they would have you believe. Young people are smart. As time goes on, congregations (in the US) will become smaller and smaller.
2. Fewer males able to teach. Thanks to Number 1, there are fewer males taking the lead. The elders and even many of the ministerial servants now are fifty or sixty years old. Existing men are being overworked and there is no relief coming. Unless the Watchtower suddenly allows for females to start teaching, in about ten years there aren't going to be enough brothers to successfully carry out all of the work.
3. Science. Science and atheism are enormous problems for the JWs right now. They no longer publish science-like articles in the Awake! anymore. This is because, with a couple of searches on Google or Wikipedia, the Watchtower can immediately be called on their bull$hit. Immediately. This isn't 1981 when it took some effort to get down to the library where we likely wouldn't even find the sources that the Awake! used in their articles. This is also why they haven't had another edition of that lame Creation book since 1984 (or whenever it came out). It is becoming more difficult to reconcile God's creation with "how the universe actually works" so they will now begin ignoring it.
As a side note, I have heard three separate talks (one at a convention) where they did not deny the creation of the universe in a massive explosion. They admitted that the universe is 14.7 billion years old, based on current scientific evidence. However, they do deny a random "big bang". The energy that caused the explosion would have had a First Cause, which was Jehovah. I had mixed feelings about the comment, but it's evident to me that some science is difficult to refute. It was their way of saying that they are not "behind the times" when it comes to science. They even acknowledged that the Earth took billions of years to form. But they're not budging on the "God created Adam from dust" business.
My point, though, of all this is that the WBTS seems to be losing its grip - on everthing. Once the infrastructure of congregations starts to crumble, there won't be much else to do. Their only alternatives will be to:
1. Allow sisters to teach. They will do this as a sign of progression but in reality it is a sign of desperation.
2. Do like many other religions have done and stop denying science and evolution. If you cannot sufficiently explain why Jehovah would give man vestigal tail bones and ear muscles, you need to let go of the fairy tale. Allow for God to have caused the "big bang", but don't tell me that Eve was made from a spare rib.
3. Stop driving away young people (and many older ones) with unscripturally rigid laws and regulations that don't even make sense 3500 years after they were written.